1. Field of the Invention
The present invention generally relates to a radiation diagnostic apparatus, an X-ray computed tomography apparatus, and an image processing method for radiating radiation rays onto an examined subject and generating an image by detecting radiation rays that have passed through the examined subject. The present invention specifically relates to generating a subtraction image by performing a dual energy subtraction process.
2. Description of the Related Art
As a method for treating the coronary arteries of the heart, a method called Percutaneous Coronary Intervention (PCI) is conventionally known. PCI is a method by which a catheter is inserted into a blood vessel of an examined subject (hereinafter, the “subject”) so that stenosed parts or obstructed parts of the coronary arteries can be expanded by using the inserted catheter. When performing a PCI, doctors bring a catheter to the location of a lesion in the coronary arteries while looking at an X-ray image taken by an X-ray diagnostic apparatus.
Further, according to a commonly-used PCI method, a contrast agent is administered to the subject so that an X-ray image can be taken by performing Digital Angiography (DA) or Digital Subtraction Angiography (DSA). DA is an image taking method used for generating an image in which blood vessels are highlighted by a contrast agent by performing digital processing. DSA is an image taking method used for generating an image in which the picture contrast of only the blood vessels is enhanced by defining an image obtained before a contrast agent is administered to the subject as a “mask image” and applying a subtraction process to the mask image and to another image obtained after the contrast agent is administered to the subject (see, for example, JP-A 2007-195633 (KOKAI)).
When an image taking method such as DA or DSA described above is used, although the image in which the blood vessels are highlighted can be generated, other sites (e.g., bones) that are not necessary in the diagnosing process are also shown in the image while overlapping the blood vessels. As a result, in some situations, it is difficult to see the movement of the heart muscles, which need to be looked at in treatments using PCI.
To cope with this problem, in recent years, an image taking method called “dual energy subtraction”, which utilizes the fact that different sites have different X-ray absorption rates, has been used. The dual energy subtraction method is an image taking method by which two sets of X-rays having mutually different levels of energy are alternately radiated onto a site that is a target of the image taking process so that a subtraction process can be applied to the obtained X-ray images. When such a dual energy subtraction method is used, it is possible to obtain an image from which the sites that are not necessary in the diagnosing process are eliminated or an image in which the contrast agent or a stent is highlighted. In the following sections, an image that is obtained by performing a subtraction process will be referred to as a “subtraction image”.
The conventional technique described above, however, has a problem where it is not possible to efficiently generate the subtraction images by performing the dual energy subtraction process in relation to the number of times the X-rays are radiated.
More specifically, according to a commonly-used dual energy subtraction method, a subtraction image used in a diagnosing process is generated by applying a subtraction process to an odd-numbered-frame image that has been taken as a (2m−1)'th image and an even-numbered-frame image that has been taken as a 2m'th image (where m is a natural number). For example, the frame rate at which the images are taken is expressed as “fr1” (images per second), the frame rate at which the images generated through the subtraction process are displayed can be expressed as “fr1”/2 (images per second). In other words, according to the conventional dual energy subtraction method, it is possible to obtain only half as many processed images as the number of times the X-rays are radiated.